Plate heat exchangers of this kind are frequently used as evaporators for evaporation of refrigerants circulated in refrigeration systems. Such a refrigeration system normally includes a compressor, a condenser, an expansion valve and an evaporator, all coupled in series. A plate heat exchanger used as an evaporator in a system of this kind often has heat transfer plates which are welded or brazed together, but also gaskets may be used as sealing means between the heat transfer plates. Normally, the passages present between the inlet channel and the evaporation flow paths between the heat transfer plates have the same size.
A problem recognized in connection with a refrigeration system of the above said kind, using a plate heat exchanger as an evaporator, is that refrigerant entering the said inlet channel of the plate heat exchanger is not evenly distributed to the various evaporation flow paths between the heat transfer plates. One reason for this may be that the refrigerant, which when entering the inlet channel is already partly evaporated after having passed through the expansion valve, does not remain in the form of a homogeneous liquid/vapour mixture along the whole of the inlet channel but tends to partly separate into streams of liquid and vapour, respectively.
Uneven distribution of refrigerant to the different evaporation flow paths in the plate heat exchanger leads to ineffective use of certain parts of the plate heat exchanger, in which parts the refrigerant is also unnecessarily overheated.
To avoid the problem of uneven distribution of the refrigerant in a plate heat exchanger of the above mentioned kind it has been suggested in the Swedish Patent Application No. 8702608-4, that a restriction means should be arranged in each passage between the inlet channel of the plate heat exchanger and each plate interspace forming an evaporation flow path, as defined above, for the refrigerant. The restriction means could be a ring or a washer provided with a hole and being arranged between adjacent pairs of the heat transfer plates. Alternatively, the restriction means could be a pipe having several holes and being arranged in the inlet channel of the plate heat exchanger. As a further alternative the restriction means could be formed integral with the heat transfer plates. Thus, plate edge portions delimiting the inlet ports of two adjacent heat transfer plates could be folded to abutment against each other, edge to edge, except in small areas forming inlet openings for the refrigerant to the flow paths formed between the adjacent plates.
Plate heat exchangers having restriction means of the kind just described are difficult to manufacture. Use of separate rings or washers is far too expensive and it is difficult to locate the rings or the washers in correct positions when a plate heat exchanger is to be assembled. A restriction means in the form of a pipe must be adapted as to its length to the number of heat transfer plates included in the plate heat exchanger and must also be correctly positioned relative to the inlet passages leading into the flow paths between the heat transfer plates. Folding of port edge portions of the plates has proved unpracticable, depending on the fact that the heat transfer plates are mostly produced from very thin sheet metal, and it is difficult to obtain well-defined inlet openings leading into the plate interspaces in the way suggested in said Swedish patent application.
DE 4422178 shows a distribution device for a two-phase refrigerant flow in a plate heat exchanger, including a hollow body with porous walls arranged in the inlet channel of the plate heat exchanger. The porous body has a central channel receiving the two-phase refrigerant coming from an expansion valve at the inlet of the plate heat exchanger and conducting it through the porous body along said inlet channel. Preferably, it is said, the porous body is tapered from the inlet end of the inlet channel and is surrounded by a sleeve having throttle openings opposite to the respective passages leading to the evaporation flow paths between the heat transfer plates. A disadvantage with this distribution device is that it is expensive and must be adapted to the length of the inlet channel.
WO 94/14021 also shows a plate heat exchanger to be used as an evaporator in a refrigeration system. A distributor in the form of a perforated tube is arranged in the refrigerant inlet port channel of the plate heat exchanger. The distributor may include flow regulating means. Also this refrigerant distribution device is expensive and disadvantageous in that it must be adapted to the length of the inlet port channel of the plate heat exchanger.